1. Field of the Invention
The present invention relates to a pulper for comminuting and suspending fibrous material, including a vat and at least one rotor arranged in the vat for circulating a fibrous material suspension located in the vat, as well as at least one rigid screen, wherein the rotor passes over at least the greatest portion of the screen surface which is provided with round screen openings.
2. Description of the Related Art
Pulpers of this type are predominantly used to bring dry pulp or recovered paper of greatly different compositions into suspension. They consist substantially of a vat for the suspension and at least one rotor. The furnished material—loose in large pieces, bales or pressed bales—is intensively mixed with water, whereby a hydraulic stock circulation is produced. Understandably such devices are optimized, whereby essentially fast and energy efficient pulping is strived for. In many cases the rotor is located in close proximity to a flat screen, thus keeping it free of clogging. The pulped stock is sorted from coarse contaminants through the screen openings and is drawn off as a suspension.
A vertically positioned cylindrical vat with a rotor in the floor area has prevailed as a standard for a pulper. In this vat, water and the stock to be pulped are added from the top and, with the assistance of the rotor a funnel, flow is produced in the suspension, whereby therefore the stock is drawn downward by the rotor and is pushed radially outward in the floor region, resulting in the rotational flow.
An additional typical design is the pulper vat which is open on top and has a side-mounted rotor. In this case the suspension is moved approximately horizontally to the rotor and diverted on the sidewall of the vat on which the rotor is mounted. This hydraulically often less favorable form has the advantage to be able to be accommodated under the paper machine as a broke pulper.
The aforementioned screen serves initially to undertake a separation whereby already sufficiently degraded portions of the paper stock are drawn off through the screen openings, and still insufficiently degraded stock is held back. This makes the pulping process more economical in a known method. If desired, the pulper may in particular be operated continuously. Contaminants, namely essentially non-paper components are held back or respectively sorted.
In many cases however, the screen also contributes to strengthening the pulping process. The side of the rotor facing the screen plate interacts with the inlet edges of the screen openings. A certain distance between rotor and screen plate is always maintained, however the shear forces occurring hereby are sufficient to degrade the paper stock particles further. Larger pieces can also bridge the distance between screen and rotor, so that a direct cutting action occurs. In what form and intensity these processes occur depends on the requirements and the design of the pulper. It is also known that the just described pulping action can be further strengthened in that the screen plates on the infeed side are equipped with bars, as known for example from DE 101 16 367.
A screen plate is known from WO 03/033152 A1 whose openings were produced by laser or water jet. These openings are not round, but are provided with corners and are distributed preferably in a mosaic-like pattern on the screen plate. This is intended to strengthen the pulping action. The known screens for pulpers of this type are usually provided with circular bores, or as described in EP 1679403 with elongated screen openings, whereby a compromise must be found between the demand for optimum classification, namely sorting openings which are as small as possible, and the requirement of an as high as possible throughput, in other words an as open as possible surface. The characteristic of such a screen results essentially from size, shape and number of screen openings disposed therein.
In addition to a high throughput, high resistance against the hydraulic pressure is also strived for. In order to meet this requirement, DE19547585 suggests a screen with a supporting- and sorting layer. However, the screen openings remain problematic, in particular to the production, throughput and risk of clogging.
What is needed in the art is a pulper having a high throughput at an as low as possible energy consumption and an as large and effective cutting edge length as possible.